Zero
A zero, when referring to firing a rifle or SMG (and sometimes, a pistol), is the setting for the sights at which the point of aim matches the point of impact at a predefined distance. Sight systems Iron sights Iron sights are zeroed by adjusting for the range (usually by adjusting the height of the rear sight), the windage (has been done on both the rear sight and front sight, so this is dependent on the firearm), and the elevation settings (usually by adjusting the height of the front sight, if the front sight is adjustable). The sights are first centered and a control group fired to determine the firearm's point of impact. Once the point of impact is determined, the sights are adjusted so that the point of aim matches the point of impact, and a shot group is fired to verify the settings at the range that the sights are to be zeroed. Red dot sights Red dot sights are zeroed by first centering the dot in the sight, and firing a control group at the range that the red dot is to be zeroed. Once the point of impact has been established relative to the point of aim, adjust the dot so that the point of aim matches the point of impact, and fire again to verify. Telescopic sights Telescopic sights are zeroed much the same way as red dot sights; however, the reticule is not a dot, and telescopic sights are meant to be fired at much longer ranges than red dots. The reticule is centered, and a control group is fired at the range desired for a zero to establish a point of impact. The optic is then adjusted to match the point of impact, and another group is fired to verify the zero. Notes No two firearms, even if they are the same type (for instance, two AR-15s from the same company built to the same specifications) will have identical settings for a zero. The zero will also vary according to the weight and powder load of the ammunition used; it is best to zero a firearm with the same ammunition that is to be used in that particular firearm. If an optic is moved from one firearm to another, it must be re-zeroed. Holdovers Once a weapon is zeroed, the sights normally shouldn't be adjusted; if the shooter is willing to note the settings or mark them somehow, and it is possible to do so, then adjustments can be made. The zero must still be confirmed once the sights are set. In most cases, when the weapon is zeroed, the point of impact will match the point of aim at only two specific distances; once when the bullet crosses the line of sight during the initial rise (near zero) and again when the bullet drops below the line of sight (far zero), after the apex of the trajectory. In select cases, the near zero and far zero will be nearly identical, due to the apex of the bullet's trajectory being very close to the line of sightA 100 yard or 100 meter zero on any AR variant (or other firearm with the line of sight approximately 2.5 inches over the bore) chambered in 5.56x45mm NATO or .223 Remington will achieve this effect. All cartridges have one zero that does this; the distance will vary due to each bullet having a different trajectory, sights resting at different heights relative to the bore, and barrel length., causing the bullet trajectory to intersect the line of sight twice within a very short distance (less than 5-10 yards/meters). Since the bullet always travels in a parabolic trajectory once fired due to gravity, actual point of impact at other distances will not match the point of aim. At distances beyond the near zero distance, but closer than the far zero distance, the point of impact will always be higher than the point of aim, with the highest point of impact at the apex of the bullet's trajectory. At distances closer than the near zero intersection, the point of impact will be lower than the point of aim. At distances beyond the far zero, the point of impact will be lower than the point of aim, and will get progressively lower at the distance increases beyond the far zero intersection. In order for the shooter to make the bullet go where it needs to go, the shooter may need to change his point of aim and account for other outside factors (such as wind, elevation, barometric pressure, and the particular load for the cartridge fired) in order for the point of impact to be where the shooter needs it to be. References Category:Terminology